1. Field of the Invention
The invention relates to a system for converting fuel and air into reformate with a reformer which has a reaction space, a nozzle for supplying a fuel/air mixture to the reaction space, and a fuel feed for supplying fuel to the nozzle.
2. Description of Related Art
Generic systems are used to convert chemical energy into electrical energy. For this purpose fuel and air, preferably in the form of a fuel/air mixture, are supplied to the reformer. In the reformer, conversion of the fuel with atmospheric oxygen takes place, preferably with the process of partial oxidation being carried out.
The reformate which has been produced in this way is then supplied to a fuel cell or a stack of fuel cells, electrical energy being released by the controlled reaction of hydrogen, as a component of the reformate, and oxygen.
The reformer, as already mentioned, can be designed such that the process of partial oxidation is carried out to produce the reformate. In this case, when using diesel as the fuel, it is especially useful to carry out prior reactions before partial oxidation. In this way long-chain diesel molecules can be converted into short-chain molecules with a “cold flame.” This ultimately promotes reformer operation. In general, a gas mixture, which is reacted to produce H2 and CO, is supplied to the reaction zone of the reformer. Another component of the reformate is N2 from the air, and depending on the air ratio and the temperature, optionally CO2, H2O and CH4 as well. In normal operation, the fuel mass flow is controlled according to the required output, and the air mass flow is adjusted to an air ratio in the range of λ=0.4. The reforming reaction can be monitored by different sensors, for example, temperature sensors and gas sensors.
In addition to the process of partial oxidation, it is also possible to carry out auto-thermal reforming. The process of partial oxidation, in contrast to auto-thermal reforming, is caused by the oxygen being supplied sub-stoichiometrically. For example, the mixture has an air ratio of λ=0.4. Partial oxidation is exothermal so unwanted heating of the reformer can occur in a problematical manner. Furthermore, partial oxidation tends to intensify soot formation. To prevent soot formation, the air ratio λ can be chosen to be smaller. This is achieved so that some of the oxygen used for oxidation is made available by water vapor. Since oxidation with water vapor proceeds endothermally, it is possible to adjust the ratio between the fuel, oxygen and water vapor such that overall heat is neither released nor consumed. The auto-thermal reforming achieved in this way eliminates the problem of soot formation and undesirable overheating of the reformer.
Additionally, it is possible for further steps of gas treatment to proceed following oxidation in the reformer, in particular, methanization can be conducted downstream of the partial oxidation.
A current fuel cell system, for example a PEM system (“proton exchange membrane”), can typically be operated at temperatures between room temperature and roughly 100° C. Based on the low operating temperatures, this type of fuel cell is often used for mobile applications, for example in motor vehicles.
In addition, high temperature fuel cells, so-called SOFC systems (“solid oxide fuel cell”), are known. These systems work in the temperature region of roughly 800° C., a solid electrolyte (“solid oxide”) being able to take over transport of oxygen ions. The advantage of these high temperature fuel cells over PEM systems is principally in the areas of durability relative to mechanical and chemical loads.
The application for fuel cells, in conjunction with generic systems, includes not only stationary applications, but also applications in the motor vehicle field, for example as “auxiliary power units” (APU).
For reliable operation of the reformer, it is important to supply the fuel or fuel/air mixture in a suitable manner to the reaction space of the reformer and to condition the fuel or fuel/air mixture suitably in the reaction space. For example, good mixing of the fuel and air and a good distribution of the fuel/air mixture in the reaction space of the reformer are advantageous for the operation of the reformer. Within the framework of the present invention, a fuel-air mixture is referred to if substances are added or will be added to the reaction space of the reformer. The added substances are not limited, however, to the mixture of fuel and air, but rather other substances can also be added, for example water vapor in the case of auto-thermal reforming. To this extent, the concept of a fuel/air mixture should be understood in this general form.